Move code in isPaintOpaque from SkDeferredCanvas.cpp to SkPaint

Move code in isPaintOpaque from SkDeferredCanvas.cpp to SkPaintPriv

The purpose of this code move is to make it re-usable in order to implement
occlusion culling optimizations in SkPicture similar to what we have now
in SkDeferredCanvas.

BUG=https://code.google.com/p/chromium/issues/detail?id=164530
Committed: https://code.google.com/p/skia/source/detail?r=7361

[junov, 2013-01-23 20:33:15]

PTAL

[reed1, 2013-01-23 20:50:41]

Lets move it into a private .h file for now (similar to our SkTSort.h or
pictureutils). I am all for sharing code, but am reluctant to add to our public
API with a compelling reason to do it ... publicly.

[junov1, 2013-01-23 21:03:34]

Right now, code in src/utils can only include the public headers from the skia
core, so that's a problem.

[reed1, 2013-01-23 21:05:57]

We should change that (it would seem). If we are supporting the features in
utils (like we do in effects/) they should have access to private resources.

[junov, 2013-01-23 21:42:49]

Patch

[reed1, 2013-01-23 22:12:26]

lgtm

[reed1, 2013-01-23 22:12:50]

https://codereview.appspot.com/7196046/diff/4001/src/core/SkPaintPriv.h
File src/core/SkPaintPriv.h (right):

https://codereview.appspot.com/7196046/diff/4001/src/core/SkPaintPriv.h#newco...
src/core/SkPaintPriv.h:15: 
// What happened to my nice dox comment?